Regarding a heat exchanger used in a heat pump hot-water supply system, a refrigerant (e.g. carbon dioxide) is used as heat source for heating water. The heat exchanger needs durability to withstand a high-temperature and high-pressure refrigerant. Recently, to maintain the durability, it is proposed to provide refrigerant passages in a plurality of capillary tubes, such as copper pipes with the diameter approximately a few millimeter, closely arranged in parallel. This kind of heat exchanger is for example disclosed in U.S. Pat. No. 6,540,015 (JP-A-2002-31488).
According to the heat exchanger of U.S. Pat. No. 6,540,015, the capillary tubes are used for defining passages of the high-pressure refrigerant. By this, effective condensation improves because of  the small diameter. A passage of the water is formed in a flat box shaped tube that are formed by joining two plates, which are produced by drawing. An inner fin is housed in the box shaped tube and the capillary tubes for the refrigerant are layered on the outer periphery of the box shaped tube. These members are made of steel products, and thereby integrally brazed.
However, the water passage in the tube defines a single flow that serpentines from an inlet to an outlet of the tube. Since the flow of water makes a lot of turns (e.g. about 100 turns), it is likely to increase resistance of the flow of the water.